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Correlating Biochemical And Chemical Oxygen Demand Of Influents And Effluents - A Case Study Of Selected Wastewater Treatment Plants In The Gaza Strip, Palestine

Congress: 2015
Author(s): Salem Gharbia, Francesco Pilla, Abdalkarim Gharbia
Trinity College, Dublin1, Institute of Water and Environment, Al-Azhar University- Gaza2

Keyword(s): Sub-theme 11: Key vulnerabilities and security risks,

The Gaza Strip is a part of the Palestinian coastal plain located in an arid to semi-arid region. It is bordered by Egypt from the south, the green line from the North, Nagev desert from the East and the Mediterranean Sea from the West, so The Gaza Strip is located on the south-eastern coast of the Mediterranean Sea, between longitudes 34° 2" and 34° 25" east, and latitudes 31° 16" and 31° 45" north. The Gaza Strip is divided geographically into five Governorates: Northern, Gaza, Mid Zone, Khan Younis and Rafah.
There are four wastewater treatment plants (WWTP) operating in the Gaza Strip: Beit-Lahia WWTP in the north, Gaza WWTP in the Gaza City, Khanuonis and Rafah WWTP in the south. The existing three WWTPs are heavily overloaded as the actual flow far exceeds the design flow. Blocked pipes and flooded manholes are daily events in Gaza Strip. The total effluent of the existing three WWTPs is approximately 41 MCM/year. The Mediterranean Sea represents the final disposal of most treated or partially treated wastewater in Gaza strip.
This work aims at establishing an empirical correlation between Biochemical Oxygen Demand (BOD5) and Chemical Oxygen Demand (COD) of influents and effluents from selected wastewater treatment plant in the Gaza strip to facilitate speedy quality assessment or optimal process control.

Materials and Methods
Monthly effluent samples were collected for three times for analysis of the principal parameters BOD5 and COD using the "Standard Methods for the Examination of Water and Wastewater". Measured BOD5 and COD of the influents and effluents were above the required discharged guideline of 30 mg/l and 100 mg/l respectively specified by the Palestinian Water Authority (PWA, 2012).
Five-Day Biochemical Oxygen Demand (The Dilution Method)
The dissolved oxygen content of liquid was determined by the Azide modification of the Winkler's method before and after incubation for five days at 200C. The difference gave the BOD5 of the sample after allowance had been made for the dilution, if any, of the sample. For optimum biochemical oxidation, the pHs of the samples for analysis were 6.5 to 8 (APHA, 1992).
Chemical Oxygen Demand (COD) Using the Open Reflux Method
The sample, to be measured, was oxidized under reflux with a known amount of potassium dichromate in strong sulphuric acid with silver sulphate as a catalyst. Organic matter reduced part of the dichromate and the remainder was determined by titration with iron (II) ammonium sulphate or iron (II) sulphate using ferroin as indicator. Interferences from chloride were suppressed by the addition of mercuric sulphate to the reaction mixture. The chemical oxygen demand (COD) was expressed as milligrams of oxygen absorbed from standard dichromate per liter of sample (APHA, 1992).

Results and Discussion
The BOD5 variation of the influent from the three selected wastewater treatment plants ranged from 330 mg/l to 800 mg/l and the range for effluent was 60 mg/l to 150 mg/l. The corresponding COD for influent ranged from 800 mg/l to 1655 mg/l and the range for effluent was 155 mg/l to 388 mg/l. These levels of BOD5 and COD of the wastewaters could pose potential pollution to the seawater in which they are discharged since untreated.
A linear response was obtained between the data sets. The fitted equations on the entire data (COD and BOD5) set on a particular industry are generated with their corresponding correlation coefficients (r). The Results indicate that the degree of common variation between the two variables COD and BOD5 is highly positive for the influents and effluents from the three selected wastewater treatment plant, thus the variables are said to be correlated. The fitted equations for the influents were (Bait-Lahia, y = 2.6207x -- 78.183, r = 1.00; Rafah, y = 3.5758x + 1131.9, r = 0.93; Khanuonis, y = 2.7143x + 98.28, r = 0.973) and for effluents were (Bait-Lahia, y = 2.6142x -- 5.0373, r = 0.991; Rafah, y = 2.3262x + 27.029, r = 0.79; Khanuonis, y = 2.0643x + 42.429, r = 0.974) with y and x representing COD and BOD5 respectively.
The correlation coefficients of the fitted equations were generally positive for the results from the three-selected wastewater treatment plant's samples analysis, being between 0.79 and 1.00. The COD and the BOD5 results obtained for the three-selected wastewater treatment plant showed that the degree of common variation between the two variables was highly positive; thus the COD and the BOD5 are said to be highly correlated.

The degree of common variation between the COD and the BOD5 of the wastewaters from the selected wastewater treatment plant in the Gaza strip (Bait-Lahia, Rafah and Khanuonis) were highly positive, thus the two parameters are correlated with good correlation coefficients of 0.995, 0.86 and 0.974 respectively. The correlation coefficient of the fitted equation for prediction for Bait-Lahia, Rafah and Khanuonis wastewater treatment plants, may therefore be used to facilitate rapid quality assessment or optimal process control once the chemical oxygen demand (COD) is measured or vice versa.

1. APHA. 1992. Standard Methods for Water and Wastewater Examination. 18th Edition. American Public Health Association, Washington, D.C.

2. Palestinian Water Authority (PWA), 2012. Hydrogeological data bank department, Gaza Strip.
2011 IWRA - International Water Resources Association - - Admin